Alloy memory

ABSTRACT

Alloy memory structures and methods are disclosed wherein a layer or volume of alloy material changes conductivity subsequent to introduction of a electron beam current-induced change in phase of the alloy, the conductivity change being detected using current detection means such as photon-emitting P-N junctions, and being associated with a change in data bit memory state.

BACKGROUND OF THE INVENTION

[0001] The predominant mass storage device in conventional computingdevices is the hard disk drive. Hard disk drives are relatively large,electromechanical devices that can store a relatively large amount ofdata. The stored data is accessed through a read/write head that rideson a cushion of air above the rapidly rotating disk. The read/write headmoves radially to access data in different tracks of the rotating disk.Data transfer is limited by the speed at which the disc rotates and thespeed with which the read/write head is positioned over the requiredtrack. Even with the fastest devices, access times are on the order ofthousands of microseconds, due to the relatively large mechanicalmotions and inertia involved. This time scale is at least seven ordersof magnitude slower than the sub-nanosecond time scales at whichprocessors operate. The discrepancy may leave the processor starved fordata. Compact Disc and DVD storage systems, also limited by the speed atwhich a disc rotates and the speed with which a read/write head ispositioned over a required track, are associated with similardiscrepancies.

[0002] During the time the processor is starved for data, eithervaluable computing time is lost or the processor must perform anothertask, which also may lead to data starvation. Such data starvedconditions are referred to in the art as being input/output (I/O) boundor bottlenecked.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The present invention is illustrated by way of example and is notlimited in the figures of the accompanying drawings, in which likereferences indicate similar elements. Features shown in the drawings arenot intended to be drawn to scale, nor are they intended to be shown inprecise positional relationship.

[0004]FIG. 1 illustrates an alloy memory element.

[0005]FIG. 2 illustrates an alloy/LED configuration.

[0006]FIG. 3 illustrates an alloy/LED memory device.

[0007]FIG. 4 illustrates an alternative embodiment of an alloy/LEDmemory device.

DETAILED DESCRIPTION

[0008] In the following detailed description of embodiments of theinvention, reference is made to the accompanying drawings in which likereferences indicate similar elements, and in which is shown by way ofillustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the invention is defined only by the appended claims.

[0009] Devices and methods are disclosed to store data using a memorywithout moving parts. In one embodiment of the present invention, anelectron beam (e-beam) is used to irradiate a volume or layer of analloy material which corresponds to an encoded data bit location.Exposure of the alloy material to a modulated high-energy e-beam changesthe conductivity of the alloy material, creating at least two differentstates of conductivity, which may be associated with the binary data bitmemory states of, e.g., “0” and “1”, or any multilevel value between thetwo terminal states. Reading the stored data is accomplished with alower energy e-beam that does not alter the state of conductivitycreated by the high-energy e-beam during data writing. The lower energye-beam may be used according to various embodiments of the invention toread the conductivity of the alloy material, which corresponds to thedata stored therein. The invention is not limited to storing and readingbinary data. The invention is applicable to n-ary data, however tosimplify the description, the example of binary data will be followed.Mass-related issues with moving parts, such as inertia, have previouslylimited the data seek time in conventional devices, which is associatedwith rotational latency and mechanical steering of a read/write head.

[0010] The association of the binary values for data bit memory states,e.g., “0” or “1”, is completely arbitrary with respect to the state ofconductivity within the alloy; the invention is not limited by theassociation made. An extension to n-ary data may be achieved by creatingmore than two distinct conductivity states within phase change material.For example, three distinct conductivity states could be associated withtrinary data.

[0011] With reference to FIG. 1, device 100 includes a volume of alloy110 coupled with a conductor 112 and a conductor 114. A variable-energye-beam 120 operating at a high-energy level, incident upon the alloy110, is used to deliver an electron current and thereby heat to thealloy 110. Use of the term “high-energy” is application-dependent andwill depend upon several device design parameters. A non-exclusive listof device design parameters influencing the term “high-energy” includesthe alloy material, the degree of conductivity change desired, thee-beam exposure time (corresponding to the time allotted to change theconductivity of the alloy), and the volume of the data cell within thealloy. A typical range of “high-energy” is 400-10,000 electron volts(eV) with an electron-beam current in the range of 10-3,000 nano-amps.

[0012] The alloy 110, shown in FIG. 1, may correspond to a data cellwithin an array of data cells. The term “alloy” generally refers to amixture of at least two metals. Similar binary or n-ary resistancebehaviors may be created in organic or organo-metallic materials andthese materials may be applied in the same way at different power levelsand cycle times for data storage. It is well known that through thermaltreatment, the microstructures of certain alloys may be manipulated toproduce stable physical states with physical, chemical, and electricalproperties that vary from other stable physical states of the same alloythat have been through different thermal treatment. Particularlyrelevant to this invention are the significant difference in electricalconductivity which exists between alloy states, and the thermaltreatment requirements to change an alloy from one state to another.Chalcogenide alloys, preferred in this invention, are known to havestable crystalline and amorphous states with significantly differentelectrical conductivities and adequate phase-reversal cyclingcapabilities.

[0013] A volume of chalcogenide alloy may be heated above its meltingtemperature and then cooled past its recrystallization temperatureeither to a primarily amorphous state (“amorphous state” or “amorphousphase”), or to a state having more crystallinity (“crystalline state” or“crystalline phase”), depending upon the cooling time during which thealloy remains above the recrystallization temperature (the “coolingprofile” of temperature versus time). To keep the material fromrecrystallizing during cooling and resulting in a crystalline phasestructure, the cooling rate must be faster than the crystal nucleationand growth rate for the particular material. Relatively fast coolingprofiles, therefore, are more likely to result in amorphous statestructures, and relatively slow cooling profiles are more likely toresult in crystalline state structures. Controlling the cooling profile,therefore, is a key variable for determining which logic state iswritten. As with other alloys, the electrical conductivity ofcrystalline phase chalcogenide alloy is significantly higher than thatof amorphous phase chalcogenide alloy due to the differences in physicalorder of the alloy material at the microstructural level and the factthat electrons travel more efficiently through ordered, or crystalline,microstructures. In terms of data bit memory state logic, the meltingmay be used to “reset” a data bit, and the cooling profile selected toeither result in a crystalline phase solid (either a “0” or a “1” in abinary example) or an amorphous phase solid (the binary complement ofthe crystalline phase logical representation). While multiplechalcogenide alloys with two or more stored phases accessible via e-beamirradiation are known, those comprising Germanium, Antimony, andTellurium are preferred, such as the alloy having the formula“Ge₂Sb₂Te₅”, with a melting temperature of 616 degrees Celsius and arecrystallization temperature of 142 degrees Celsius. With thispreferred alloy, an amorphous phase may be reached along a relativelyfast cooling profile by cooling past the recystallization temperaturewithin about two nanoseconds, while a crystalline phase may be reachedby a relatively slow cooling to the recrystallization temperature in aminimum of about two nanoseconds. Also preferred are alloys such asthose used in high-speed optical storage devices for “fast writing”,such as those comprising combinations of Germanium, Antimony, andTellurium, and combinations of Silver, Indium, Antimony, and Tellurium,wherein a crystalline phase of relatively high conductivity may bereached as a result of heating the fast-write material to about 85% ofthe melting temperature for the composition, followed by relatively slowcooling to enable heated material to nucleate relatively large,contiguous crystals based upon adjacent crystallites of the samematerial positioned around the boundary of the heated region. Suchfast-write alloys enable a faster change from amorphous phase tocrystalline phase, since the material need not be taken to the meltingtemperature before crystal nucleation. To reach an amorphous phase ofrelatively low conductivity with a fast-write material, heating to about110% of the melting temperature for the material is followed by coolingalong a relatively fast cooling profile, wherein the cooling rate isfaster than the crystal nucleation and growth rate for the particularmaterial to avoid the formation of a highly-ordered crystallinestructure. Therefore, as opposed to melting the alloy for a “reset” typeoperation, and cooling at various rates to proceed to either acrystalline phase, or an amorphous phase, as with conventionalchalcogenide alloys, fast-write alloys need not be melted unless anamorphous phase result is desired—since a crystalline phase may bereached by heating to about 85% of the melting temperature, followed bycooling along a relatively slow cooling profile. For example, afast-write alloy associated with cooling profiles similar to thosedescribed above in reference to the preferred Germanium, Antimony, andTellurium alloy, may be heated to about 85% of melting temperature andcooled to the recystallization temperature of the fast write alloy in aminimum of about two nanoseconds to provide a highly conductivecrystalline structure, while the same fast write material may be heatedto about 110% of melting temperature and cooled past itsrecrystallization temperature within about two nanoseconds to provide aless conductive amorphous structure.

[0014] E-beam irradiation is used both as a means for heating the alloy110 to melting temperature, and also as a means for controlling thecooling profile of temperature versus time as the alloy is cooledsubsequent to melting. In one embodiment, for example, a reset andsubsequent amorphous stored phase is achieved by e-beam irradiation tomelting followed by cooling without further e-beam irradiation, while areset and subsequent crystalline stored phase is achieved by e-beamirradiation to melting followed by a decrease in e-beam irradiationalong a slower cooling profile. In other words, a crystalline storedphase is reached via a cooling profile wherein cooling is slowed bycontinued irradiation at a decreased rate. In one embodiment, a decreasein irradiation to slow cooling is achieved using power tapering, whereinthe voltage and/or current to the e-beam source is decreased. In anotherembodiment, intermittent exposure, or exposure tapering, is used todecrease irradiation during cooling. In another embodiment, a slowercooling profile is achieved by creating thermal mass in an adjacentstructure during cooling. Such thermal mass may be achieved using e-beamexposure. Hybrids of the aforementioned cooling profile controltechniques may also be utilized. For example, the cooling at a data celllocation may be controllably slowed by irradiating structuresimmediately surrounding the location to create thermal mass around thelocation, while the location itself also may be irradiated to a degreesufficient to contribute to slow cooling of the location. Manyvariations of exposure patterns may be employed in such hybridembodiments, including but not limited to spiral patterns, wherein aportion of the data cell location at the center of the spiral receivesmore focused irradiation than other portions, due to the focusing natureof a repeated spiral pattern and/or decreased radial spacing betweenspirals as they get closer to the portion of the data cell location atthe center of the spiral; stepped concentric circles used in a similarfashion as described for spirals, with the exception that the e-beam ispaused at each of a set of circular radii, the radii being equallyspaced or concentrated toward the center of curvature; and rectilinearscanning or rastering combined with focused irradiation exposure to atargeted location. Similarly, hybrid exposure patterning could be usedto emphasize exposure more upon the adjacent structures as opposed tothe data cell location itself. Exposure patterning may be combined withpower tapering and/or exposure tapering to provide further coolingprofile control. Cooling profiles for chalcogenide alloys such asGe₂Sb₂Te₅ are known in the art and used in devices such as CD-RW, whichuse laser light irradiation rather than e-beam irradiation to controlcooling.

[0015] Referring back to FIG. 1, in a data write phase, electron e-bearn120 (at high-energy) creates a change in conductivit within the volumeor layer of alloy 110. Reading the data is accomplished by introducingcurrent into the alloy 110 via a low-energy e-beam, from either thewrite e-beam source modulated to a lower energy or from an alternatee-beam at a lower energy, than the energy level of the “high-energy”e-beam used for writing the data. This low-energy e-beam used forreading may be less than approximately 100 electron volts (eV), andshall be of a sufficiently small diameter so that it will not degradethe data bit memory state of the alloy at the data bit location or anyadjacent data bit locations. In other words, the low-energy e-beamirradiation for reading data causes an excitation of the alloy 110insufficient to heat the alloy 110 above its recrystallizationtemperature.

[0016] The low-energy e-beam introduces an electron current into a databit location corresponding with device 100 in FIG. 1. The conductor 112and reference conductor 114 provide a differential conducting path,which will conduct a majority of the current indicated by 122 (the“bleed off current”) when the alloy 110 is highly conductive.Alternatively, when the alloy 110 is highly resistive an increase willoccur in the number of the electrons that will propagate down throughthe alloy 110 and reference conductor 116 to the conductor 114. Theconductor 114 and the reference conductor 116 provide a seconddifferential conducting path for the current indicated by 124 (the“transmitted current”). Either one or both of the bleed-off andtransmitted currents may be used to associate the state of conductivitywithin the alloy 110 with the memory state of data stored in a bit. Thee-beam 120 depicted represents either the high-energy e-beam used towrite the data or the low energy e-beam used to read the data.

[0017] Once the current has been steered, according to the state of thealloy 110, many embodiments of the present invention may be used tosense, i.e. read the current. In one embodiment of the presentinvention, a fixed impedance reference layer (typically with animpedance much greater than the low impedance memory state of the alloystorage layer) is attached to the alloy on the side opposite to theelectron-beam. The electronbeam's current distribution is then measureddifferentially between the conductor 112 and the conductor 114 using acurrent detector (not shown). The current will distribute differentlybased upon the conductivity of the alloy storage layer.

[0018] In another embodiment, the reference layer is replaced by using arelatively thick layer of the alloy where only the portion of this layerclosest to the electron-beam 120 is thermally modified to represent thememory state, while the remaining alloy material remains unchanged fromthe initial condition, thus acting as the reference layer cited in thepreviously-described embodiment.

[0019] In another embodiment of the present invention, the device 100(FIG. 1) may be coupled with a light-emitting semiconductor P-N junctionas shown in FIG. 2 to achieve a differential light emission device 200corresponding to the state of conductivity within the alloy 110. FIG. 2illustrates an alloy/LED configuration. With reference to FIG. 2, a P-Njunction 211 is placed beneath phase change material 110. The P-Njunction 211 may be created from a direct band semiconductor such asthose commonly made from III-V elements, e.g., GaAs, or other electronexcited light emitting structures. In one embodiment, doping is arrangedso that the N-type 210 layer of the P-N junction 211 is coupled with thealloy 110 allows for easy transport of the filtered electrons into theP-N junction 211. A very thin, conductive interlayer may be used tobackward bias the P-N junction from a high impedance source or toprovide lattice matching between the P-N junction and the alloy 110.Below a P-type 212 layer of the P-N junction, a conductor 214 is placedwhich may be a normal metal pad. The conductor 214 supplies holes to theP-N junction. The material used for the conductor 214 may be selected tooptimize the reflection of the light generated in the P-N junction. Anadvantage of using a direct-band semiconductor is that the recombinationof electrons and holes may create photons without requiring phononemission to conserve momentum. The thickness of the N-type 210 layer andthe P-type 212 layer must be sufficient to support the full transitionregion and optimally couple to external electromagnetic modes.

[0020] In another embodiment of the present invention, differentiallight emission is achieved by the device 200 by varying the amplitude ofthe current passed by the varying conductivity of the alloy 110 betweenthe cross-linked and damaged states. In one conductivity state 218, thealloy 110 will conduct an increase in the number of electrons throughits thickness to the P-N junction resulting in a maximum emission oflight. The maximum emission of light may correspond to one memory statefor the data stored therein. The emitted light may be sensed byphoto-detector 224. In another conductivity state 216, a minimum amountof current is conducted through the alloy 110, instead the majority ofthe current is “bled off” via conductor 112, as previously describedresulting in a minimum emission of light from the P-N junction. Theminimum emission of light may correspond to a second memory state forthe data stored therein.

[0021] The alloy/P-N junction structure, shown at 200, need not beetched out from the surrounding material to allow photons to escape theP-N junction 211. The layer (conductor 112) above the P-N junction maybe made very thin. Photons can penetrate more than a micron of metalconductor thickness and much further through other materials. Therefore,the conductor 112 may be on the order of a micron or less in thickness,thereby providing a sufficient electrically conductive path whileallowing photons to pass through the conductor 112. The device 200 actsas a tiny dot illuminator when irradiated with the read e-beam, causingthe P-N junction to emit light in one conductivity (memory) state whilethe P-N junction remains dark in another conductivity (memory) state.

[0022] An embodiment of the present invention is shown in FIG. 3illustrating an alloy/P-N junction memory device. With reference to FIG.3, one or more alloy data elements are indicated at an alloy/LED 324array. A vacuum shroud or enclosure 326 and an end cap 312 form a closedcontainer (a high vacuum environment) in which electron beam source 314emits e-beam 120, incident upon the alloy/LED 324 planar array. Controlelectronics 310 may be used in conjunction with the electron beam source314 as needed to control the e-beam source. The e-beam 120 may besteered by means of electron lens 316 and deflection electrodes 318.

[0023] The e-beam 120 may be used to write data to the alloy/LED 324planar array, as previously described, as well as to read data writtentherein. Accordingly, the level of light generated by the alloy/P-Njunction of the data bit is measured by a sensitive photo-detector 320,by methods well known in the art. The vacuum shroud 326 may be madereflective on the inside, thereby acting as an integrating sphere forthe emitted photons, which increases the signal-to-noise ratio for themeasurement made by the photo-detector 320. The output of thephoto-detector 320 may be amplified as required by photo amp 322. Thee-beam 120 is steered across the phase change material/LED 324 array toread data stored in the data bit locations corresponding to dots on thesurface of alloy/LED 324. Thus, writing and reading data is accomplishedwithout the mechanically articulated parts required by the hard diskdrive, CD-ROM and the DVD. Using the teachings of the present invention,the seek time to reach any block of data is on the order is tens ofmicroseconds.

[0024] A read-after-write capability is provided during data bit writingby the conductivity state switching within the alloy 110. During datawriting with the high-energy e-beam, the impedance of the alloy 110 willchange—due to the induced state change, resulting in a sudden pulse oflight as electron current is passed or removed from the P-N junction.Sensing the pulse of light during the thermal decrease cycle willprovide a read-after-write capability similar to that provided by thesecond head in a tape drive or similar polarization shift effect in amagneto-optic disc drive.

[0025] In an alternative embodiment, the substrate (conductor 214 inFIG. 2) may be made sufficiently thin or transparent to allow photons tobe emitted from the opposite side that e-beam 120 is incident upon phasechange material 110. This opposite side is indicated as 220 in FIG. 2and FIG. 3. The emitted light may be sensed from the lower side of thestructure as shown in FIG. 2 at photo-detector 224 a and in FIG. 3 atphoto-detector 320 a. The photo-detector 320 a may be configured withphoto-amp 322 a. This arrangement has the feature of removing theelectron optics from the photo-detector's field of view. One way toprovide a high vacuum environment to the lower side of vacuum shroud 326is to couple fixture 326 a with vacuum shroud 326 by mating as indicatedat 330 a and 330 b.

[0026] The e-beam 120 may be on the order of 20 nanometers in diameter.It is possible to steer the e-beam 120 through an angle of approximately20 degrees, as will be explained in conjunction with FIG. 4. Very smalle-beam sources may be manufactured, made using silicon processes. Anexample of such a device is the electron micro-column made by theSensors Actuators and Microsystems Laboratory (SAMLAB), which is part ofthe Institute of Microtechnology, University of Neuchatel, located inSwitzerland. These very small e-beam sources (micro-column e-beamsources) may be produced in a form factor measuring approximately threemillimeters wide and approximately one millimeter high. In oneembodiment, using these parameters, a data storage device may be builtto store approximately a terabyte of data on a polymer/LED area ofseveral square centimeters.

[0027] The present invention may be incorporated into various memorydevices. FIG. 4 shows an alternative embodiment of a memory device 400.With reference to FIG. 4, a cylindrical container 412 is shown having aphase change material/LED layer 324 and an electron beam source 314. Thee-beam source 314 may be steered through an angle 410 as shown. Thus,data may be written to an array having approximately 200 gigabytes ofdata in a device occupying a volume of approximately several cubicinches. These data storage devices may be configured in an array toachieve terabyte data storage capacities.

[0028] It is expected that many other shapes and configurations of datastorage devices are possible within the teachings of the presentinvention. For example, a cube may be configured (not shown) with phasechange material arrays lining the interior surfaces thereof. One or moreelectron beam sources may be configured within the cube, each facing aninterior surface of the cube and being capable of writing and readingdata stored in each phase change material array.

[0029] Thus, a novel solution to electron beam recording and sensing ofdata bits is disclosed. Although the invention is described herein withreference to specific preferred embodiments, many modifications thereinwill readily occur to those of ordinary skill in the art. Accordingly,all such variations and modifications are included within the intendedscope of the invention as defined by the following claims.

1. A method to store a data bit comprising: exposing a volume of alloyto an electron beam to melt the alloy; cooling the volume of alloy alonga cooling profile associated with a stored phase and stored data bitmemory state, to a temperature below the recrystallization temperaturefor the volume of alloy, to achieve the stored phase.
 2. The method ofclaim 1 wherein the volume of alloy has a first stored phase and asecond stored phase, the first stored phase being associated with arelatively fast cooling profile and a relatively low conductivity, thesecond stored phase being associated with a relatively slow coolingprofile and a relatively high conductivity.
 3. The method of claim 2wherein the relatively fast cooling profile comprises cooling past therecrystallization temperature for the volume of alloy within 2nanoseconds.
 4. The method of claim 2 wherein the relatively slowcooling profile comprises cooling to the recrystallization temperaturefor the volume of alloy in a minimum of 2 nanoseconds.
 5. The method ofclaim 1 wherein cooling the volume of alloy along a cooling profilecomprises further exposing the volume of alloy to the electron beam toreduce the rate of cooling of the volume of alloy.
 6. The method ofclaim 1 wherein cooling the volume of alloy along a cooling profilecomprises exposing structures adjacent the volume of alloy to theelectron beam to reduce the rate of cooling of the volume of alloy. 7.The method of claim 1 wherein cooling the volume of alloy along acooling profile comprises exposing both the volume of alloy andstructures adjacent the volume of alloy to the electron beam to reducethe rate of cooling of the volume of alloy.
 8. The method of claim 5wherein further exposing comprises exposing at a decreased level ofirradiation using power tapering.
 9. The method of claim 5 whereinfurther exposing comprises exposing at a decreased level of irradiationusing exposure tapering.
 10. The method of claim 7 wherein exposing boththe volume of alloy and structures adjacent the volume of alloy to theelectron beam comprises exposure patterning.
 11. The method of claim 2wherein the volume of alloy comprises a chalcogenide alloy.
 12. Themethod of claim 11 wherein the chalcogenide alloy comprises an elementfrom the group including Germanium, Antimony, and Tellurium.
 13. Amethod to read a data bit comprising: exposing a volume of alloy to anelectron beam; detecting a state of conductivity within the volume ofalloy; associating the state of conductivity with a data bit memorystate.
 14. The method of claim 13 wherein exposing a volume of alloy toan electron beam causes an excitation of the volume of alloyinsufficient to heat the volume of alloy above its recrystallizationtemperature.
 15. The method of claim 13, wherein detecting a state ofconductivity within the volume of alloy comprises using a differentialconducting path to detect quantities of current moving through andaround the volume of alloy.
 16. The method of claim 13, wherein currenttransmitted through the volume of alloy interfaces with a P-N junctionlayer to emit photons in response to current transmitted through thevolume of alloy; and wherein detecting a state of conductivity withinthe volume of alloy comprises monitoring photons emitted from the P-Njunction layer.
 17. The method of claim 13 wherein the volume of alloycomprises a chalcogenide alloy.
 18. The method of claim 17 wherein thechalcogenide alloy comprises an element from the group includingGermanium, Antimony, and Tellurium.
 19. A device to store a data bitcomprising: a volume of alloy having a first side and a second side; afirst conductive material disposed on the first side; and a secondconductive material disposed on the second side; wherein an electronbeam irradiated upon the volume of alloy, having changed the crystallinemicrostructure in the volume of alloy, changes the conductivity of thevolume of alloy.
 20. The device of claim 19 wherein the volume of alloycomprises a chalcogenide alloy.
 21. The device of claim 19 furthercomprising a reference conductor, the reference conductor being inelectrical contact with the first conductive material and the secondconductive material.
 22. The device of claim 21 further comprising acurrent detector to detect current in the first conductive material. 23.The device of claim 21 further comprising a current detector to detectcurrent in the second conductive material.
 24. The device of claim 20wherein the chalcogenide alloy comprises an element from the groupincluding Germanium, Antimony, and Tellurium.
 25. A device to read adata bit comprising: a volume of alloy having a first side and a secondside; a first conductive material disposed on the first side; a P-Njunction disposed on the second side; an electron beam source, togenerate an electron beam incident upon the first side of the volume ofalloy to cause an emission of photons from the P-N junction; and aphotodetector responsive to the emission of photons, wherein an outputof the photodetector is associated with a data bit memory state.
 26. Thedevice of claim 25 further comprising a substantially transparent layercoupled with the P-N junction, to transmit the emission of photonsthrough the substantially transparent layer.
 27. The device of claim 25further comprising a reflective layer coupled with the P-N junction, toreflect the emission of photons back through the P-N junction.
 28. Thedevice of claim 25 wherein the volume of alloy comprises a chalcogenidealloy.
 29. The device of claim 25 wherein the P-N junction comprises adirect band semiconductor.
 30. The device of claim 25 further comprisingan vacuum enclosure to encapsulate the electron beam source and thevolume of alloy in a vacuum.
 31. The device of claim 26 wherein the P-Njunction is positioned between the electron beam source and thephoto-detector.
 32. The device of claim 27 wherein the photodetector ispositioned between the electron beam source and the P-N junction. 33.The device of claim 28 wherein the chalcogenide alloy comprises anelement from the group including Germanium, Antimony, and Tellurium. 34.The device of claim 25 further comprising a lattice-matching layerdisposed between the P-N junction and the volume of alloy.
 35. A deviceto read a data bit comprising: a volume of alloy having a first side anda second side; a first conductive material disposed on the first side; aP-N junction disposed on the second side; a second conductive materialdisposed on the P-N junction; a reference conductor coupled with the P-Njunction; and an electron beam source to generate an electron beamincident upon the volume of material to create a first current to bemeasured between the first conductive material and the referenceconductor, and a second current to be measured between the secondconductive material and the reference conductor.
 36. The device of claim35 wherein the volume of alloy comprises a chalcogenide alloy.
 37. Thedevice of claim 36 wherein the chalcogenide alloy comprises an elementfrom the group including Germanium, Antimony, and Tellurium.
 38. Thedevice of claim 35 wherein the P-N junction comprises an N-type layer,the N-type layer being coupled with the volume of alloy.
 39. The deviceof claim 35 wherein the P-N junction is a direct band semiconductor. 40.The device of claim 35 further comprising a thin conductive layerdisposed between the P-N junction and the volume of alloy to backwardsbias the P-N junction.
 41. The device of claim 31 further comprising athin conductive layer disposed between the P-N junction and the volumeof alloy to perform lattice matching between the P-N junction and thevolume of alloy.
 42. A method to store a data bit comprising: exposing avolume of fast-write alloy to an electron beam to heat the volume offast-write alloy to a temperature equivalent to about 85% of the meltingtemperature for the fast-write alloy; cooling the volume of fast-writealloy along a cooling profile associated with a crystalline phase and astored data bit memory state.
 43. The method of claim 42 wherein coolingthe volume of fast-write alloy along a cooling profile comprises coolingto the recrystallization temperature for the volume of fast-write alloywithin 2 nanoseconds.
 44. The method of claim 42 wherein cooling thevolume of fast-write alloy along a cooling profile comprises furtherexposing the volume of fast-write alloy to the electron beam to reducethe rate of cooling of the volume of fast-write alloy.
 45. A method tostore a data bit comprising: exposing a volume of fast-write alloy to anelectron beam to heat the volume of fast-write alloy to a temperatureequivalent to about 110% of the melting temperature for the fast-writealloy; cooling the volume of fast-write alloy along a cooling profileassociated with a amorphous phase and a stored data bit memory state.46. The method of claim 45 wherein cooling the volume of fast-writealloy along a cooling profile comprises cooling past therecrystallization temperature for the volume of fast-write alloy within2 nanoseconds.
 47. The method of claim 45 wherein cooling the volume offast-write alloy comprises cooling along a cooling profile associatedwith an amorphous phase.